The traditional, image forming apparatus comprises a motor, a driving head for an image forming apparatus and a process cartridge, wherein the process cartridge is detachably mounted in the image forming apparatus and provided with a photosensitive drum and a photosensitive drum driving head fixedly connected to the end portion of the photosensitive drum. During the operation of the image forming apparatus, the power is generated by the motor and transmitted to the process cartridge via the engagement between the driving head for the image forming apparatus and the photosensitive drum driving head, so as to make the photosensitive drum on the process cartridge rotate.
As illustrated in FIGS. 1 and 2, the photosensitive drum driving head is fixedly arranged at one end of a photosensitive drum 7 and comprises a convex connection shaft 17 provided with a twisted projection 17a having an end surface 17a1; a rotation center of the convex connection shaft 17 and a rotation center of the photosensitive drum 7 are aligned; and a concave connection shaft 18 comprise a twisted recess 18a and a bottom surface 18a1 is provided on the recess 18a. 
During the operation of the image forming apparatus, the driving head 18 for the image forming apparatus receives the rotary power from the motor; the convex connection shaft 17 is engaged with the driving head 18 for the image forming apparatus; and the rotary power is transmitted to the convex connection shaft 17 through the driving head 18 of the image forming apparatus and finally makes the photosensitive drum rotate. When the convex connection shaft 17 is engaged with the driving head 18 of the image forming apparatus, the twisted projection 17a on the convex connection shaft 17 is interposed into the twisted recess 18a in the driving head 18 of the image forming apparatus; the end face 17a1 is directly opposite to the bottom surface 18a2; and the rotary power on the driving head 18 of the image forming apparatus is transmitted to the convex connection shaft 17 via the engagement between the twisted projection 17a and the recess 18a. 
FIGS. 3 and 4 are sectional views respectively illustrating the state when the twisted projection 17a and the twisted recess 18a do not rotate and rotate. As shown in the figures, both cross sections of the twisted projection 17a and the twisted recess 18a take the shape of triangles (such as equilateral triangles), and the dimension of the triangular projection 17a is less than that of the triangular recess 18a. As illustrated in FIG. 3, when the projection 17a is interposed into the recess 18a but does not rotate along with the recess 18a, a rotation axis X1 of the convex connection shaft on the photosensitive drum is not aligned with a rotation axis X2 of the driving head of the image forming apparatus. As illustrated in FIG. 4, when the projection 17a is engaged with the recess 18a and rotates along with the recess 18a, three vertex angles 17a2 of the triangular projection 17a are engaged with three edges of the triangle on the recess 18a, and the power is transmitted to the twisted projection 17a from the recess 18a, and the rotation axis X1 of the convex connection shaft on the photosensitive drum is aligned with the rotation axis X2 of the driving head of the image forming apparatus, so that the stable transmission between the twisted projection 17a and the twisted recess 18a during the operation can be guaranteed. In the figure, R0 refers to the diameter of a rotation circle of the three vertex angles 17a2 of the projection 17a; R1 refers to the diameter of an inscribed circle of the triangular recess 18a; and R2 refers to the diameter of a rotation circle of three vertex angles of the recess 18a. In order to achieve the power transmission between the twisted projection 17a and the recess 18a of the image forming apparatus, R0, R1 and R2 must satisfy the condition of R1<R0<R2.
FIG. 5 illustrates another embodiment of the prior art. In the embodiment, both the twisted projection 17a and the twisted recess 18a take the shape of quadrilaterals (such as squares) and are engaged with each other for power transmission.
In the prior art, the driving head of the image forming apparatus may also adopt the mode as illustrated in FIG. 6. As illustrated in FIG. 6, a twisted recess 28a is formed at one end of a driving head 28 of the image forming apparatus and provided with a bottom surface 28a1 and a projection 28a2 which is disposed at the center of the twisted triangular recess 28a (a rotation center of the boss is aligned with a rotation axis X2 of the driving head of the image forming apparatus). In addition, the height of the projection 28a2 is substantially the same with the depth of the recess 28a, and the projection may be conical.
The photosensitive drum with the photosensitive drum driving head is widely used in a process cartridge for the traditional image forming apparatus. The process cartridge at least comprises a photosensitive drum, a developer and a developing roller, wherein the photosensitive drum used for forming an electrostatic latent image is provided with the photosensitive drum driving head; the developer is used for developing the electrostatic latent image; and the developing roller is used for transmitting the developer to the photosensitive drum. When the process cartridge is mounted into the image forming apparatus for use, the rotary power from the motor of the image forming apparatus is received by the driving head of the image forming apparatus, so as to make the photosensitive drum and the developing roller rotate.
The power transmission structure in the prior art has the defects that:
1. When the twisted projection is engaged with the twisted recess, the twist angles of twisted surfaces on the projection and the recess ask for high accuracy. In the case of inconsistent twist angles of the twisted surfaces on the projection and the recess due to the problem of the manufacturing accuracy, the twisted surface on the projection makes point-to-surface contact with the twisted surface on the recess, and one of the twisted surfaces may be deformed during the engagement between the projection and the recess, so that the rotation axis X1 of the convex connection shaft on the photosensitive drum cannot be aligned with the rotation axis X2 of the driving head for the image forming apparatus, and hence the stability of power transmission may be affected. In order to avoid the above problem, the manufacturing accuracy of the twisted surfaces of the projection and the recess must be very high, and thus the manufacturing cost can be increased and the problems of difficult manufacturing and the like can be caused.
2. As the polygonal shapes of the projection and the recess are difficult to process, the projection and the recess ask for high manufacturing accuracy. Taking an equilateral triangle for example, the accuracy of centers of triangles can only be guaranteed under the condition of high accuracy requirement on the equilateral triangle projection and the equilateral triangle recess, or else, the rotation axis X1 of the convex connection shaft on the photosensitive drum may be not aligned with the rotation axis X2 of the driving head for the image forming apparatus when the projection and the recess are engaged with each other, so that the instable transmission may be caused. Moreover, during the engagement between the projection and the recess, the vertex angles of the triangle on the projection, for the reason of power transmission, tend to be deformed due to the application of force and are vulnerable to wear or damage during long term operation. Furthermore, the three vertex angles of the triangle simultaneously have the functions of rotating due to the application of force and supporting and positioning during the operation, so that the centers X1 and X2 tend to be not aligned with each other during the engagement between the worn or damaged triangle and the recess, and thus the transmission stability may be affected. Therefore, in order to guarantee the accuracy and stability of transmission, the requirements on the hardness and wear resistance of materials of the triangular projection are very high. Similarly, the positions, making contact with the three vertex angles of the projection, on the edges of the triangular recess are also vulnerable to wear or damage during the operation, so that the requirements on the hardness and wear resistance of the triangular recess are also high.